The styles which come with Word 2007 are not really structured, you will have to dig into the code to find what you need. You have one thing which works in your advantage. Only the author and the year are in bold (HTML element
b). Locate the section between
<!-- Bunch of code. -->
It seems there are two parts there:
<!-- Code taking care of the author/title part. -->
<xsl:if test ="string-length(b:Year)>0">
Without testing it, it looks like removing that second part might be enough to fix your style.